P3D TDC Overheating problems

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

I went back and checked the postings around December 1996.  We had
long discussions on this subject back then.  Here is a summary of my
measurements and observations.

First, I established the "softening point" of the RBT mounts.  Using
various heating methods I concluded that the RBT plastic mounts start
flowing at around 110 C. (I will be using degrees C for the rest of
this discussion;  for reference, the boiling point of the water is 
100 C which is 212 F)  Certainly, under 90 C the RBT mounts maintain
their structural integrity.

After that, I used a temperature probe placed at the film gate to measure
the temperature under various combinations (500W bulbs, 750W bulbs, heat
absorbing filters ON, OFF, metallic finish or black finish).

Here is a summary of the measurements:

Condition                 500W bulbs        750W bulbs
---------------------------------------------------------------------
Heat Filters ON            43 / 60            50 / 82
Heat Filters OFF           65 / 125           77 / >150

The first value listed is the temperature for the metal (bare) probe, 
simulating what an aluminum mask should feel.  The second value
listed is the temperature for the probe covered with black plastic,
simulating the black side of an RBT mount.

The problem of overheating slides or mounts in projection has to do with
the IR radiation from the lamps.  The purpose of the heat absorbing
filters is to cutoff a good part of the IR radiation that causes heat
built up.  Black surfaces tend to absorb IR radiation very quickly.
>From the measurements above we see that if the heat filters are there
then even the black side of the RBT mounts should not melt within a
reasonable time.  BUT I PERSONALLY RECOMMEND THAT THE WHITE SIDE OF
THE RBT MOUNTS ALWAYS FACES THE LAMPS!  No sense in inviting heat
built-up by having the black side facing the lamps.

Without heat filters you are in trouble!  The heat filters are 
flat and have a greenish cast.  They are the only flat pieces of
glass inside the TDC projector and they are placed in between
two pairs of condenser (curved) lenses.  They can be removed
(easier in a 116 model).  Take them out and inspect them.  I have 
seen heat filters in 716 models that are covered with some kind of
deposit and the glass is etched.  Try and clean them.  
Alan.Hancock@xxxxxx posted information on heat filter replacement
back in December 1996.  See also good postings in behalf of Colin
Clay from UK.

*** Tip of the day ***************************************************
Tip:  To test the condition of the heat filters just place an RBT
black mount *in front* of the lenses.  If it melts, you have a problem.
If the filters are OK then the mount should be OK.  Air circulation 
is not an issue there because there isn't any air circulation outside
of the projector.
**********************************************************************

George Themelis


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